U.S. patent number 7,663,064 [Application Number 11/162,720] was granted by the patent office on 2010-02-16 for high-speed flex printed circuit and method of manufacturing.
This patent grant is currently assigned to Banpil Photonics, Inc.. Invention is credited to Achyut Kumar Dutta, Robert Olah.
United States Patent |
7,663,064 |
Dutta , et al. |
February 16, 2010 |
High-speed flex printed circuit and method of manufacturing
Abstract
Multilayer high speed flex printed circuit boards (FLEX-PCBs)
are disclosed including a dielectrics systems with the back-side
trenches, adhesives, signal lines and ground planes, wherein the
signal line and ground plane lane are located on the dielectrics.
Using of the open trenches in the substrate help to reduce the
microwave loss and dielectric constant and thus increasing the
signal carrying speed of the interconnects. Thus, according to the
present invention, it is possible to provide a simply constructed
multiplayer high speed FLEX-PCB using the conventional material and
conventional FLEX-PCB manufacturing which facilitates the design of
circuits with controlled bandwidth based on the trench opening in
the dielectrics, and affords excellent connection reliability. As
the effective dielectric constant is reduced, the signal width is
required to make wider or the dielectric thickness is required to
make thinner keeping fixed characteristics impedance. The
fundamental techniques disclosed here can also be used for
high-speed packaging.
Inventors: |
Dutta; Achyut Kumar (Sunnyvale,
CA), Olah; Robert (Sunnyvale, CA) |
Assignee: |
Banpil Photonics, Inc. (Santa
Clara, CA)
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Family
ID: |
37884776 |
Appl.
No.: |
11/162,720 |
Filed: |
September 20, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20070066126 A1 |
Mar 22, 2007 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60522401 |
Sep 25, 2004 |
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Current U.S.
Class: |
174/261; 29/830;
174/255 |
Current CPC
Class: |
H05K
1/024 (20130101); H05K 3/386 (20130101); Y10T
29/49126 (20150115); H05K 2201/09727 (20130101); H05K
3/4611 (20130101); H05K 3/4635 (20130101); H05K
3/4697 (20130101); H05K 1/0393 (20130101); H05K
2201/09536 (20130101) |
Current International
Class: |
H01R
12/04 (20060101); H05K 1/11 (20060101) |
Field of
Search: |
;174/255,261
;29/830 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Patel; Ishwarbhai B
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Patent
Application No. 60/522,401 filed on Sep. 25, 2004.
Claims
What is claimed is:
1. A multi-layered flex printed circuit board (Flex-PCB)
comprising; at least one first dielectric layer having a first
surface and an opposing second surface; at least one first
electrical signal line located on said first surface of said at
least one first dielectric layer; at least one first backside open
trench located on said second surface of said at least one first
dielectric layer, located under and aligned with said at least one
first electrical signal line; at least one first adhesive layer
located under said second surface of said at least one first
dielectric layer; at least one second dielectric layer having a
first surface and an opposing second surface; at least one first
ground/power plane located on said first surface of said at least
one second dielectric layer; at least one second backside open
trench located on said second surface of said at least one second
dielectric layer; at least one second adhesive layer located under
said second surface of said at least one second dielectric layer;
at least one third dielectric layer having a first surface and an
opposing second surface; at least one second electrical signal line
located on said first surface of said at least one third dielectric
layer, located under and aligned with said at least one second
backside open trench; at least one third backside open trench
located on said second surface of said at least one third
dielectric layer, located under and aligned with said at least one
second electrical signal line; at least one third adhesive layer
located under said second surface of said at least one third
dielectric layer; at least one fourth dielectric layer having a
first surface and an opposing second surface; and at least one
second ground/power plane located on said first surface of said at
least one fourth dielectric layer; thereby forming a stacked
arrangement of dielectric layers, wherein said at least one first
dielectric layer is stacked above said at least one second
dielectric layer, said at least one second dielectric layer is
stacked above said at least one third dielectric layer, and said at
least one third dielectric layer is stacked above said at least one
fourth dielectric layer, and wherein said at least one first
electrical signal line and said at least one first ground/power
plane form at least one microstrip type transmission line, and
wherein said at least one first ground/power plane, said at least
one second electrical signal line, and said at least one second
ground/power plane form at least one stripline type transmission
line, and wherein the structure of said backside trenches is
adapted to reduce effective dielectric constant and/or dielectric
loss.
2. The printed circuit board according to claim 1, wherein the
cross sectional shapes of the backside open trenches are selected
from the group consisting of a circle, an ellipse, and a polygon
having greater than 4 sides, convenient for manufacturing.
3. The printed circuit board according to claim 1 further
comprising a dielectric layer having at least one ground/power line
and at least one signal line on the same surface, thereby forming a
coplanar type transmission line, wherein said microstrip type,
stripline type, and coplanar type transmission lines are
single-ended or differential-ended.
4. The printed circuit board as claimed in claim 3, wherein the
coplanar type transmission line is exposed to either in dielectric
media or in air media.
5. The printed circuit board as claimed in claim 1, wherein said
fourth dielectric layer is homogeneous.
6. The printed circuit board as claimed in claim 1, wherein said
fourth dielectric layer is homogeneous, and wherein said at least
one second backside open trench and said at least one third
backside open trench are further aligned with said at least one
first electrical signal line.
7. The printed circuit board as claimed in claim 1, wherein the
trenches are filled with a material of lower dielectric loss and/or
lower dielectric constant than the dielectric layers.
8. The trench as claimed in claim 1 is filled up or coated by
liquid crystal polymer to have tunability of the dielectric loss
(or dielectric constant).
9. The printed circuit board according to claim 1, wherein at least
one of the signal lines comprises: at least one first section
located on a portion of its respective dielectric layer having back
side open trench; and at least one second section located on a
portion of its respective dielectric layer having no back side open
trench, wherein said at least one first section is wider than said
at least one second section; and wherein the printed circuit board
comprises at least one via that connects at least two different
layers of the printed circuit board connected to said second
section.
10. The printed circuit board according to claim 9, further
comprising a dielectric system having open trench located above the
first at least one section of the first signal line.
11. The printed circuit board as claimed in claim 1, wherein the
adhesive for stacking multiple dielectric layers is either no-flow
or flow adhesive.
12. The printed circuit board as claimed in claim 1, wherein the
adhesive layers comprise acrylic adhesive or epoxy adhesive.
13. The printed circuit board as claimed in claim 1, wherein the
dielectric layers comprise epoxy glass composites, polymers
including polyimide, resin, alumina, boron nitride, silicon oxide,
aluminum nitride, low temperature or high temperature ceramics,
silicon nitride, semiconductor, or PTFE.
14. The printed circuit board as claimed in claim 1, wherein the
adhesive layers are continuous.
15. The printed circuit board as claimed in claim 1, wherein the
adhesive layers are discontinuous, such that no adhesive exists in
the opening of said trench.
16. The printed circuit board as claimed in claim 1, comprising a
rigid circuit board.
17. The printed circuit board according to claim 1, wherein the
trench is filled with coolant to cool the circuit board.
18. The flex printed circuit according to claim 1, comprising
n-number of first dielectric layers, n-number of second dielectric
layers, n-number of third dielectric layers, and n-number of fourth
dielectric layers, wherein n is an integer greater than 1,
alternately arranged to build the multilayer printed circuit
board.
19. The printed circuit board according to claim 1, further
comprising: at least two electronic elements, wherein said at least
two electronic elements are connected by at least one of the signal
lines, and at least one via connecting at least two of the
layers.
20. The flex printed circuit according to claim 19, wherein at
least one of the at least two electronic elements is selected from
a group consisting of elements used in off-chip, cable,
board-to-board, and rack-to-rack interconnect systems.
21. The flex printed circuit according to claim 19, wherein the
signal line and the ground line are further alternatively arranged
in n-number of layers building the multilayer flex printed circuit
board connecting the at least two electronic elements.
22. The multilayer high-speed flexible printed circuit board in
claim 19, further comprising at least one pad or bump for
bonding.
23. The process for building the FLEX-PCB as claimed in claim 1,
comprises; patterning, etching to form electric signal line on
first polymer; opening back-trench in said first polymer; cutting
epoxy (adhesive), stacking polymer having said signal line and
back-trench with second polymer layer having uniform metal plan in
one side, using said adhesive; wherein, said adhesive is located in
between two said polymers.
24. The process for building of the FLEX-PCB as claimed in claim 1,
with the stripline type and microstrip type signal lines,
comprises; formation of at least one signal line on at least one
polymer substrate; opening at least one back-trench in said polymer
substrate carrying said signal line; opening at least one back
trench in polymer substrates having ground plane, required on-top
of said signal line for the case of stripline type signal line,
and; stacking said polymer substrates with said signal lines or
uniform ground plane by using adhesive in between two said polymer
substrates; wherein, the stripline configuration has at least two
trench openings locating in said polymer dielectrics, close
proximity to said signal line, and wherein microstrip line has one
trench opening locating in back side of said polymer substrate.
Description
FIELD OF THE INVENTION
This invention relates to high speed electrical interconnects for
chip-to-chip interconnection, more particularly on the high-speed
flex printed circuit board (FLEX-PCB), where two or more integrated
circuits (ICs) are needed to connect each others signal lines for
communicating. These types of high speed FLEX-PCBs could be in all
kinds of computers covering from personnel computer to
super-computer, server, storage system, game system, imaging
system, and networking systems. This invention is also related to
the high-speed electrical interconnection, optical interconnection
or both electrical and optical interconnection where FLEX-PCB is
used for two or more high-speed electronics and/or optical elements
connection.
BACKGROUND OF THE INVENTION
The increasing of higher level of integration within electrical
integrated circuit (IC) leads to both higher data rates and larger
number of IC interconnections. Today, the inherent signal speed of
IC is increased to 3 GHz, and shortly it will be reached to 10 GHz
and beyond. The number of pin connection is also increased, with
single IC requiring close to 2000 interconnection (i.e. single
processor), and shortly it will be increased to over 5000.
Simultaneously achieving higher data rates and higher interconnects
densities for off-chip, will be increasingly difficult as the IC
technologies continue to evolve increasing signal speed of
electronic devices and interconnection number. In off-chip case,
high density interconnects, covering from die-level packaging to
chip-to-chip (hereafter chip indicates the die with package)
interconnection on the FLEX-PCB, will also be getting increasingly
difficult as the IC technologies continue to evolve increasing the
signal speed and interconnection number.
With increasing of the signal speed and interconnection number of
the IC, low-cost high-speed interconnect technique compatible to
today's manufacturing process are highly desirable to make
available in consumer level. Today's FLEX-PCB is mainly made of
uniform Polyimide material, and their manufacturing technology
along with FLEX-PCB manufacturing are so well matured that, for
long run, all system vendors like to use Polyimide based FLEX-PCB
to keep the system cost low. However, Polyimide has material
characteristics, which limit its usage in high speed if
conventional interconnection structure is used. The reason is that
conventional Polyimide has the high dielectric loss which mainly
limits the bandwidth of interconnects.
FIG. 1 is the schematic showing a part of conventional FLEX-PCB.
For simplicity in understanding, only a portion of the FLEX-PCB is
shown here. Conventional FLEX-PCB 10 consists of single or
multilayer of uniform core layers 12, adhesive used for attaching
signal lines and ground to Polyimide 14A and also used for stacking
multilayers 14, signal lines 16, and ground 18. The core layer 12
could be any uniform dielectric layer. Usually, Polyimide is used
as the core layers in conventional FLEX-PCB. The adhesive 14 is a
B-stage (partially cured) acrylic used in between the core layers
12 to stack the multiple core layers. Conventional FLEX-PCB is
supplied by vendors as C-staged acrylic (fully cured) adhesive 14A
attaching rolled flexible conducting material (usually copper) to
the Polyimide core material. The high-speed electrical signal flow
through the signal lines 16 attached by adhesive 14 to the core
layers 12 and the ground line 18 is laid opposite side of the core
layer 12. As shown in FIG. 2, the thickness H, core layer's
relative dielectric constant .di-elect cons., metal thickness T,
and the width W of the signal lines determine the impedance of the
signal line. The signal lines 16 can be the microstripline type
signal line 16A or stripline type signal line 16B, as shown in FIG.
2. In conventional FLEX-PCB, the microstrip line type signal line
16A in which the ground 18 is separated by the uniform/homogeneous
dielectric (core and adhesive) layer 12 and layer 14. Stripline
type signal line 16B is also used in conventional FLEX-PCB, in
which the signal line 16B is embedded into the homogeneous
dielectric (core and adhesive) layer 12 and layer 14, and both
sides the ground 18 is used.
Conventional FLEX-PCB 10 as shown in FIG. 1 is manufactured in the
way, the flow chart of which is shown in FIG. 3. This is an
explanatory diagram for the prior art of FLEX-PCB manufacturing.
The dielectric sheet (not shown) 20 is made using the standard
FLEX-PCB technology for example using the slurry casting process.
The slurry is cast into about 200 .mu.m to 500 .mu.m thick ceramic
sheets by slip cast process. The FLEX-PCB core layer 12 is the
homogeneous layer usually used in the conventional FLEX-PCB 10.
After the patterning and subsequent etching, the signal line is
made on side of the core layers. Microvia and subsequent filling
process 24 is done, if necessary. Following this, the sheets 26 are
laminated together by hot press to form the FLEX-PCB 28. Density
heterogeneities in the laminated samples influence any shrinkage in
the sintered substrate. Therefore, this lamination process is
homogenously carried out by means of the correct dimensional die
and punch with flat surfaces. Burn out and sintering process for
the multilayered FLEX-PCB board 10, may be necessary after
lamination at the temperature suitable to material used as the
sheet. The via hole opening and subsequent metal filling (not shown
here) are usually done. A sheet 20 may have more than 10,000 via
holes in a in a 50 to 500 .mu.m square area.
In conventional FLEX-PCB 10, as the signal line 16 is either laid
on the dielectric material (core layer 12) or embedded into the
dielectrics, based on the dissipation factor (tangent loss) of the
dielectric material used as the core layer in the FLEX-PCB, the
signal experiences dissipation while propagating through the signal
line 16. The reason is that the electric field starts from signal
line and ends in the ground 18 (not shown) and this electric field
passes through the dielectric. This signal dispersion is
proportional to the signal frequency, i.e., signal speed. It does
mean that the higher the signal speed, the lower the distance of
transmission of signal for the fixed dielectric material. In the
other words, the higher the speed, the lower the bandwidth of the
signal line which is used for connecting one chip to other chip on
the board. If the tangent loss of the dielectrics are high, the
bandwidth of the interconnects gets so limited that, high speed
signal can't be sent over longer distance as compared with the
dielectrics having the lower tangent loss.
In addition to tangent loss, the dielectric constant of dielectrics
material is also important, as electrical field inside dielectric
material having higher dielectric constant experiences more signal
delay as compared with that of transmission line comprising with
lower dielectric constant material. These causes signal skews for
the different length signal lines. In this case also, lower
dielectric constant material is necessary in the interconnection
for high-speed signal interconnection. This is true for both
on-chip and off-chip interconnection. Lower dielectric constant
material with low dielectric loss offers following functions;
Higher density interconnection is possible due to reduction of the
cross talk, (2) reducing the capacitance of the interconnection,
helping to transfer the signal longer distance, and (3) lower
propagation delay.
Considering signal loss and signal delay for various signal line
length it is highly desirable to design the interconnects on
FLEX-PCB with effective dielectric constant and effective loss of
the interconnect system lower.
It is very straight forward that increasing the bandwidth can be
possible using of the material having the lower loss tangent
(dielectric loss). However, in this case, for off-chip
interconnection new material development is necessary. Besides,
manufacturing technology is needed to develop to implement in the
product level. Conventionally, to increase the interconnects
bandwidth, dielectrics having lower tangent loss is used as the
FLEX-PCB layer. This dielectric material is very high cost and the
manufacturing process for building FLEX-PCB using these materials
are not matured yet. In addition, the FLEX-PCB made of such low
loss material has low reliability. It is highly desirable to have
high speed FLEX-PCB that can be built up with the conventional
well-matured dielectric material (for example Polyimide) and also
conventional well-matured fabrication process can be used. This can
not only reduce the cost, but also have high reliability.
Much work can be found in off-chip interconnection technology
focusing on the material development. As for example, low loss
materials like Rogers R/flex 1100, etc. are under development
stage, to achieve high bandwidth. Implementing new material in
FLEX-PCB fabrication process will cost tremendously to make it
mature. In addition, new materials having low tangent loss is a
material incompatible with conventional dielectric material such as
Polyimide processing so is not a low cost solution. These materials
will require a much higher temperature and pressure for lamination.
Today, in developing the high speed FLEX-PCB, more focused are
being paid on shortening the length or on the interconnection
layout. In both cases, implementing technology would need to pay
high cost.
As explained above, the conventional FLEX-PCB technology being used
for off-chip interconnection cannot be used as the need of the
signal speed is increasing. And also exiting conventional
electrical interconnects have the limitation of achieving the
bandwidth in certain level, beyond that complete manufacturing
technology is needed to be changed which is costly for FLEX-PCB
industries. It is highly desirable to have lower dielectric
constant and lower dielectric loss (loss tangent) by adopt a
technique or method which can be easily implemented, and which can
use the standard dielectric material FLEX-PCB technology.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide the
technique to reduce the effective dielectric constant and effective
dielectric loss of interconnection system especially dielectric
material, to increase the bandwidth of the interconnection for
building high speed FLEX-PCB.
Accordingly, it is an object of this invention to use the
inhomogeneous dielectric system to reduce the effective dielectric
loss and dielectric constant of the dielectric material.
According to the invention it is an object to provide the
interconnection structure where large portion of the signal
(electromagnetic wave) is allowed to pass through the air or
dielectric material having the dielectric loss less than the
dielectric material itself on which the signal line is laid
out.
It is an object of this invention to provide the manufacturing
process of the high-speed FLEX-PCB carrying the high-speed signal
lines.
Another object of the present invention is to provide the
interconnection structure for inter-chip (off-chip) interconnection
on the board, which is compatible to available FLEX-PCB
technology.
According to the invention, the high speed FLEX-PCB for off-chip
interconnection comprises, (i) single or multiple electrical signal
lines for carrying the electrical signal from one electronics
elements to another and vice-versa for electrical communication;
(ii) single or multiple dielectrics which are in stacked by
adhesive wherein the dielectric system carrying the signal lines
has structure comprising with back slot or open trenches with
deepness and width, and located under the signal line (conductor);
(iii) a ground or power line located to opposite side of the
dielectrics, wherein the shape of the back-slot or trench could be
rectangular or square or circular or any shapes convenient for
manufacturing, and covering the width the same, or less or more
than the metal conductor carrying the signal.
According to this invention, the signal line of microstripline type
configuration has one open trenches under the signal lines, and the
signal lines of strip-line configuration has the opened trenches
located top and bottom of the dielectrics.
According to this invention, it is our object to provide the
structure of the opened-trenches under the signal lines of the high
speed FLEX-PCB.
According to the invention it is an object to provide the via
structure to connect the two or more layers of high speed FLEX-PCB
considering both from manufacturing point of view and also from the
impedance point of view.
According to this invention, high speed FLEX-PCB process comprises,
(i) first single or multiple core layers formation having copper
layer in only one side and B-stage acrylic adhesive on side
opposite copper; (ii) making the signal lines in single or multiple
core layers; (iii) opening the trenches in opposite side of the
signal lines, wherein the trench depth is decided from the
bandwidth required for the interconnects and the trench width can
be selected based on the convenience in manufacturing and the
requirement in interconnects bandwidth; (iv) hot press and
lamination for stacking the sheets, and; (v) sintering under
temperature.
According to this invention, the process for FLEX-PCB having the
signal line of microstrip line configuration, comprises, (i) first
single or multiple core layers formation having copper layer in
only one side and B-stage acrylic adhesive on side opposite copper;
(ii) making the signal lines in the first core layer; (iii) opening
the trenches in opposite side of the signal lines located on first
core layer, wherein the trench depth is decided from the bandwidth
required for the interconnects and the trench width can be selected
based on the convenience in manufacturing and the requirement in
interconnects bandwidth; (iv) second core layer formation having
copper layer in only one side of the core layer; (v) hot press and
lamination for stacking the first core layer, adhesive layer, and
second core layer with uniform copper layer, and; (vi) sintering
under temperature.
According to this invention, the process for FLEX-PCB having the
signal line of strip line type configuration, comprises, (i) first
core layer formation having uniform copper layer in only one side
and B-stage acrylic adhesive on side opposite copper; (ii) making
the signal lines in the first core layer; (iii) opening the
trenches in opposite side of the signal lines located on first core
layer, wherein the trench depth is decided from the bandwidth
required for the interconnects and the trench width can be selected
based on the convenience in manufacturing and the requirement in
interconnects bandwidth; (iv) second core layer formation having
uniform copper layer in only one side and B-stage acrylic adhesive
on side opposite copper; (v) opening the trenches in opposite side
of the uniform copper layer (of second core layer), wherein each
trench's position is the same as that of the trenches made in the
first core layer and the trench depth is decided from the bandwidth
required for the interconnects and the trench width can be selected
based on the convenience in manufacturing and the requirement in
interconnects bandwidth; (vi) third core layer formation having
uniform copper layer in only one side of the core layer; (vii) hot
press and lamination for stacking the second core layer with
trenches, adhesive, first core layer, adhesive layer, and third
core layer, and; (viii) sintering under temperature.
According to this invention, the electrical signal line could be
microstrip type or strip line type or coplanar type waveguide.
According to the invention, the dielectric material having lower
dielectric loss than the dielectric material on which the signal
line is drawn can fill the trench or back-slot of the dielectric
system.
According to the invention, the trench or back-slot of the
dielectric system can be filled with air or kept in vacuum.
According to the invention, the trench or back-slot of the
dielectric system can be filled by the liquid crystal material,
which can tune the dielectric constant and loss.
According to this invention, the opened trench can be filled with
the coolant to cool the FLEX-PCB.
According to this invention, the high speed communication can be
possible between two or among more than two electrical (or optical)
elements where electrical, optical or both electrical and optical
signal are used for transmission through the interconnects.
According to this invention, the effective loss tangent and
effective dielectric constant of the dielectric system is reduced,
which reduce the microwave-loss and makes to increase the
interconnects bandwidth for high speed electrical signal
propagation, and also reduce the signal propagation delay. The
lower the microwave loss to zero, the closer to be the
electromagnetic wave to the speed of the light.
The invention offers to fabricate the high speed FLEX-PCB (which
can be used to connect the signal lines of two or more IC among
each other to communicate without sacrificing each inherent signal
speed. The high speed FLEX-PCB embedding with the high speed
interconnects can be easily fabricated using conventional FLEX-PCB
manufacturing technology. The methods described in this disclosure
enables to make the electronics interconnects for inter-chip
connection in cost-effective manner and suitable for practical
application.
The other object of this invention is to minimize the skew in the
signal interconnection, occurred due to the signal propagation
delay, by reducing the microwave loss.
Other objects, features, and advantages of the present invention
will be apparent from the accompanying drawings and from the
detailed description that follows below.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be explained in more detail in conjunction with
the appended drawings wherein,
FIG. 1 is the cross-sectional view showing the prior art of the
FLEX-PCB used for electrical connection of ICs intra-chips. For
simplicity, enlarged view of a multilayerd FLEX-PCB is shown.
FIGS. 2A and 2B are the cross-sectional views' showing prior art of
electrical signal lines used in FLEX-PCB for inter-chip (off-chip)
connection;
FIG. 3 is the schematics illustrating the prior art fabrication
process of the FLEX-PCB;
FIG. 4 is the cross-sectional view showing the high speed FLEX-PCB
used for off-chip interconnects according to the inventions. For
simplicity, enlarged view of a multiplayer FLEX-PCB is shown;
FIGS. 5A and 5B are the cross-sectional views' showing the
electrical signal lines embedded into the high speed FLEX-PCB for
off-chip connection in t according to this invention;
FIG. 6 is the schematics illustrating the fabrication process of
the signal line of microstripline configuration used in the high
speed FLEX-PCB in according to this invention;
FIG. 7 is the schematics illustrating the fabrication process of
the signal line of stripline configuration used in the high speed
FLEX-PCB in according to this invention;
FIG. 8 is the schematics illustrating the fabrication process of
the high speed multilayered FLEX-PCB embedding with high speed
signals, in according to this invention;
FIG. 9 is the schematics illustrating the structure of the via
structure used in the high speed FLEX-PCB, in according to this
invention;
FIGS. 10A and 10B are the schematics illustrating the top view of
the signal lines on the high speed FLEX-PCB, in according to this
invention; The transition of signal lines layout is shown, which is
considered from manufacturing point of view and also keeping the
impedance constant along the signal line to Via.
FIG. 11 is the schematics illustrating the structure of the opened
trenches under the signal lines used in the high speed FLEX-PCB, in
according to this invention;
FIG. 12 is the graph showing the variation of the dielectric
constant and dielectric loss as a function of the dielectric
removal for trench opening, in according to this invention. This is
an explanatory diagram to show the advantage of this invention. In
this calculation, Polyimide is considered as the FLEX-PCB's
dielectric material, and also the width of the opening is
considered to be the same as that of the signal line.
FIGS. 13A and 13B are the graphs showing the variation of the
dielectric constant as function of the signal line width with
various dielectrics layers thickness as the parameters, in
according to this invention. This is an explanatory diagram for the
microstrip type signal lines, to show the advantage of this
invention. Design can be performed in various ways to get the
maximum benefits of these inventions. In this calculation,
Polyimide is considered as the FLEX-PCB's dielectric material, and
also the width of the opening is considered to be the same as that
of the signal line.
FIGS. 14A and 14B are the graphs showing the variation of the
dielectric constant as function of the signal line width with
various dielectrics layers thickness as the parameters, in
according to this invention. This is an explanatory diagram for the
stripline type signal lines, to show the advantage of this
invention. Design can be performed in various ways to get the
maximum benefits of these inventions. In this calculation,
Polyimide is considered as the FLEX-PCB's dielectric material, and
also the width of the opening is considered to be the same as that
of the signal line.
FIG. 15 is the graphs showing the bandwidth of the interconnects
for various percentage of the dielectrics removal under the signal
lines, in according to this invention. This is an explanatory
diagram to show the advantage of this invention. Design can be
performed in various ways to get the maximum benefits of these
inventions. In this calculation, Polyimide is considered as the
FLEX-PCB's dielectric material, and also the width of the opening
is considered to be the same as that of the signal line. FLEX-PCB
with 30 cm in length is also considered in the calculation.
FIG. 16A is the top view and 16B and 16C are the side and front
cross-sectional views along AA' and BB' direction of FIG. 16A,
illustrating the interchip (off-chip) interconnections consisting
of the multilayered high speed FLEX-PCB in according to the present
invention;
FIG. 17A is the top-view and 17B and 17C are the side and front
cross-sectional views along AA' and BB' directions of FIG. 17A,
illustrating mountable/stackable the interchip (off-chip)
interconnections consisting of the multilayered high speed FLEX-PCB
in according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The best modes for carrying out the present invention will be
described in turn with reference to the accompanying drawings. In
the following description, the same reference numerals denote
components having substantially the same functions and
arrangements, and duplicate explanation will be made only where
necessary.
An important point of high speed FLEX-PCB having high speed
electrical interconnects is that the microwave loss is to be
reduced by reducing the effective dielectric constant, resulting in
increasing the bandwidth of the interconnects and keeping the
signal-speed of the interconnection system closer to the source
speed. Other point is also kept in mind that the technique is to be
cost effective, and compatible to standard manufacturing technology
can be used.
In interconnects system for two or more electronics elements (two
or more ICs etc.) Connections, the signal can be conveyed
electrically through the wire (electrical conductor) laid on the
dielectric medium. For high speed signal transmission electrical
conductor is to be transmission line of type microstrip or strip
line. The signal speed in the interconnects (i.e. bandwidth of the
interconnects system) is mainly dominated by; (a) signal conductor
parameters (i) length and (ii) thickness, and (b) dielectric
material properties (i) dielectric constant, and (ii) loss tangent.
Longer interconnect length will increase the capacitance by
A.di-elect cons.L/d, where A is the area of the signal conductor,
.di-elect cons. the dielectric constant of the material, and d the
thickness of dielectric material. With optimized design,
capacitance is mostly limited by the dielectric constant. As
frequency increases the signal is started to attenuate due to the
skin effect. For example Cu at 100 GHz, the skin-depth
(.delta.)=0.2 .mu.m. For comparatively lower frequency, this
skin-depth can be neglected. Therefore, bandwidth of the
interconnect system is mainly dominated by the dielectric material
properties such as dielectric constant and loss tangent. For
increasing the bandwidth of the interconnects, their values should
be low.
It is very straight forward that increasing interconnects bandwidth
can be possible by using of the low dielectric loss material in
off-chip interconnects. However, new materials are needed and
manufacturing technologies are to be developed to implement new
material into high speed FLEX-PCB fabrication.
It is highly desirable to have the high speed FLEX-PCB, the
interconnect embedded into it should have the low effective
dielectric loss and dielectric constant and such high speed
FLEX-PCB can be fabricated using conventional manufacturing
technology.
In the preferred embodiments explanation, first the structure of
the high speed FLEX-PCB along with the techniques to reduce the
effective dielectric constant and dielectric loss, according to
this invention will be explained, and later part of this section
covers the fabrication process and some design estimations based on
conventional material such as Polyimide dielectrics as the
examples, related to the preferred embodiments.
(a) High-Speed FLEX-PCB Structure:
FIG. 4 is the cross-sectional views of the portion of the high
speed FLEX-PCB, in accordance to the present invention, wherein
like parts are indicated by like reference numerals as used
previously, so that repeated explanation is omitted here. The high
speed FLEX-PCB 20 as shown in FIG. 4 consists of the four layers of
cores 12 and adhesive 14A on which two layers are for the signal
lines 16A and 16B, and two layers for the ground (that could be
used as the power or ground) 18. These four core layers with signal
lines and ground are stacked together to form the
multilayered-FLEX-PCB 20 by using two layers adhesive 14 in between
of two layers of the cores. The signal lines 16 A is microstrip
type and signal lines 16B are in stripline type signal lines.
According to this invention, in high speed FLEX-PCB 20, the signal
lines 16A and 16B has lower dielectric loss and lower dielectric
constant, since the trenches 22 and 24 are opened under the signal
lines. The signal line 16A of microstrip type has one trench of 22
under the signal and the signal line 16B of stripline type has the
two openings, one 24A is in the top layer and other in the bottom
layer 24B. According to this invention, the effective dielectric
loss (loss tangent of dielectric system) can be reduced and the
signal attenuation while propagating can be reduced. In other
words, signal transmission is less dispersive, and higher bandwidth
of the interconnects system is ascertained, as compared with the
conventional FLEX-PCB where signal conductor is laid onto the
uniform dielectric medium, as shown in FIG. 1 as an example of the
prior art. Less cross talk is also expected as the effective
capacitance is also decreased. Based on the design, the significant
of the electromagnetic wave can be made to pass through the
open-trench 22 or 24. The width of the trench can be adjusted based
on the signal line width. The signal line impedance can be designed
by designing the trench, dielectric layer thickness, and the signal
line width. According to this invention, the opened trenches can be
filled with the air or dielectric material (not shown) having lower
dielectric loss than the dielectric material 12.
FIGS. 5A and 5B are the cross-sectional views of the portion of the
signal lines of microstrip type and stripline type configurations,
respectively, in accordance to the present invention, wherein like
parts are indicated by like reference numerals as used in FIG. 4,
so that repeated explanation is omitted here. In FIG. 5A, the
signal lines 16A is microstrip type signal line and it consists of
two layers of the core layers 12, in which the top layers 12 has
the signal lines 16A and opened trench 22 under the signal lines
and the bottom dielectric layer (core) 12 has the uniform metal
layer 18 which acts as the ground in the microstrip type signal
line 16A. In FIG. 5B, the signal lines 16B are the stripline type
signal line and it consists of three layers of the cores 12, in
which the top layers 12 has the ground 18 with opened trenches 24A,
middle layer 12 has the signal lines 16B with opened trench 24B
aligned along with signal lines 16B, and the bottom core layer 12
with the ground metal layer 18. These three layers are stacked
together to form the stripline type signal lines. According to this
invention the effective dielectric loss and dielectric constant
(i.e. microwave index) are considerably decreased as mentioned in
earlier. The explanation for getting the high bandwidth is already
explained in FIG. 4, so that related explanation is omitted
here.
According to the invention, based on the interconnect structure
design, the effective dielectric loss and effective dielectric
constant of the interconnect system can be controlled. This helps
to add many features in the interconnection such as varying the
phase velocity (which is function of the dielectric constant),
varying the bandwidth of the interconnect; help to adjust the skews
of the signal etc. in the single interconnect system. According to
the preferred embodiment, ideally, the speed of the signal in the
signal line can be made to speed of the light in the air, if other
loss due to the signal line structure such as the electrode
parameter (resistance, capacitance) are neglected. The bandwidth of
the electronic interconnect system can be possible to make the
closer or greater than optical fiber (closer to the light). In the
example, the dielectric system consisting of the opened
(backside)-trench or backside slot is considered. This invention
covers all high-speed FLEX-PCB in which embedded signal lines as
mentioned earlier are used, which is used for off-chip
interconnects.
According to this invention, the high speed FLEX-PCB can be
designed using single or plurality of dielectric layer(s) with
backside opened-trench or slot under the high-speed signal line.
For simplicity, we have shown the four layered-FLEX-PCB with having
two signal lines layers and two ground layers. However this present
invention also includes all high speed FLEX-PCB having single or
multiple layered FLEX-PCB having the trench or slot under the
signal line to increase the bandwidth of the interconnection
system.
According to the present invention, it is our object to control the
propagation of the electrical field significantly inside the trench
or slot (by filling with the air or low loss (and/or dielectric
constant) material which thereby increasing the bandwidth of the
interconnection system and reduce the signal propagation delay. In
the preferred embodiments, as explained above from FIG. 4 and FIG.
5, the strip-line and microstrip line configuration with single or
two signal lines are shown in the object of explaining the
inventions. These inventions also cover other single or multiple
signal lines in other configuration such as coplanar-line
configurations. Signal lines could be single or differential
line.
In the preferred embodiments, the dielectric layer is mentioned in
an object to cover all dielectric materials, which show the
dielectric properties. The dielectric materials include all kinds
of ceramic materials such as Duroid, AlN, Al.sub.2O.sub.3, Mullite
(3Al.sub.2O.sub.3: 2SiO.sub.2), SiC, SiO.sub.2, Silcon nitride,
silicon carbide, Silicon-Oxy-Nitride, BeO, Cordie-rite (magnesium
alumina silicate), BN, Glass (with different compositions),
polyimide, epoxy-glass, FR4, CaO, MnO, ZrO2, PbO, alkali-halide
(e.g. NaBr, NaCl) etc.) BN, BeO, and all kinds of low temperature
cofired ceramics etc., and all kinds of the polyimides and
benzocyclobutenes (BCBs) having dielectric properties. All kinds of
polymer materials having dielectric properties falls also under
this dielectric material. These dielectric materials can be made
using high temperature ceramics processing or using the IC
fabrication process. Polymer dielectric material also includes, but
not limited to, Teflon, liquid crystal polymer, epoxy, parylene,
silicone-polyimide, silicone-gel, and fluorinated ethylene
propylene copolymer. It also includes materials of elastomers (e.g.
silicone elastomer), monomers, and gels. Dielectric materials,
which can be made using high temperature ceramics processing or
using the IC fabrication process, also include this category. All
standard polymers can be available from the standard manufacturer
for example, Du-pont, Hitachi-Chemical, Mitsui, and
Mitsubishi-Chemical Industries. Gore-Tex, Japan, markets liquid
crystal polymer.
In the preferred embodiments as explained FIGS. 4 and 5, dielectric
systems having backside-opened-trench into the dielectric layer are
considered. The opened trench could be filled up with any
dielectric material having lower dielectric loss and/or lower
dielectric constant than the dielectric core layer. Alternatively,
the lower dielectric constant material can be air or vacuum.
Alternatively, in the preferred embodiment, trench or slot can be
filled up fully by the liquid crystal material or coated by liquid
crystal. The electrical field can change the orientation of the
liquid crystal and can have the control of the effective dielectric
constant and dielectric loss of the dielectric system. This could
also provide the tunability of the effective dielectric constant
and the loss of the dielectric system.
According to the present invention, the high speed FLEX-PCB is made
using the dielectric system, which has lower effective dielectric
loss and dielectric constant. The preferred embodiments can be
applied in many applications in different ways and forms. For
examples, preferred embodiments mainly can be used for high speed
FLEX-PCB where interconnects for connecting high-speed multiple
(two or more) ICs. The application includes, but not limited to,
(a) off-chip interconnects for example, connecting two or more
electronics chips on the board, (b) high speed chip (die)
packaging, and (c) high speed electrical multichannel ribbon type
flex printed circuit for connecting multiple electrical modules for
example board-to-board interconnection, rack-to-rack
interconnection, etc.
In the preferred embodiments as explained below, high speed
FLEX-PCB process is explained in an object of showing its
manufacturability using the conventional manufacturing process. (of
the techniques to reduce the microwave loss and dielectric constant
to increase the bandwidth and to reduce the signal propagation
delay), but not limited to, the specific description provided. The
design estimation is also included in an object to show the
reduction of the effective dielectric constant and effective
dielectric loss factor, and the significant improvement of
interconnects bandwidth. It is also noted here that based on the
dielectrics removal, the bandwidth of the interconnects embedded
into the high speed FLEX-PCB can also be adjusted.
(b) High-Speed FLEX-PCB Process and Design
Before going to explain the fabrication process of the multilayered
high speed FLEX-PCB with embedding the high speed signal lines
mentioned above, we will explain the process for the two main
signal lines which are microstrip type signal lines and strip line
type signal lines. The multilayered high speed FLEX-PCB may have
single or multiple layers of such signal lines embedded into the
FLEX-PCB.
(i) Fabrication Process for Microstrip Line Type Signal Lines
FIGS. 6A, 6B, 6C, and 6D are process steps for building the high
speed FLEX-PCB with microstrip type signal lines in accordance to
the present invention, wherein like parts are indicated by like
reference numerals, so that repeated explanation is omitted here.
Enlarged cross-sectional views of a portion of high speed FLEX-PCB
are only shown for explanation. In the preferred embodiment, the
process for the high speed FLEX-PCB having only microstrip lines
type signal lines consists of signal lines 16A formation from
uniform metal layer 25 of the sheet material 26, opening the
trenches 22 under the signal lines 16A, stacking with other sheet
material 28 having one side uniform metal layer which acts as the
ground 18. The stacking can be performed using the acrylic (called
as the adhesive) 30 to form the multilayered high speed FLEX-PCB 32
having microstrip line type signal lines. The trench 22 can be
opened inside the sheet material 26 by using the laser drilling or
mechanical drilling. In the case of the laser drilling case,
commercial available the Carbon-di-Oxide (CO.sub.2) laser or Nd:
YAG laser or Excimer laser or ultra-violet (UV) laser with optics
arrangements can be used. The trench deepness (Hm/n in FIG. 5,
where m and n are the integer and varies as 1, 2, 3, 4, . . . ) can
be controlled by adjusting laser intensity and pulse width of the
laser illumination. Adjusting the optics of the system can control
the width. Siemens, Germany markets the UV and CO.sub.2 lasers for
microvia fabrication. The laser technology has been matured so much
that today making via or microvia, it takes minimal time. Several
companies such as Siemens, Germany, Electro Scientific Industries,
Portland Oreg., USA etc. markets the instruments which can do fast
microvia. For example, CO.sub.2 Laser, marketed by Siemens,
Germany, can make >20,000 number of vias per min having 75 to
200 .mu.m diameter in conventional polyimide board. Most time is
the off time (shifting time) from one via to other. According to
this invention, similar laser drilling technology can be used to
open the backside trench, which is the additional process necessary
in the high speed FLEX-PCB buildup process. The time will be
shorter for continuous drilling according to this invention.
According to this invention, as a conventional dielectrics,
polyimide frequently used as the FLEX-PCB materials can be used. In
this case, CO2 laser or YAG laser can be used for drilling to open
the trench under the signal lines. According to this invention, it
is estimated that 160 inch of length of line having 8 mil
(.about.200 .mu.m) size can be made by one minute, which turns to
9600 inches/hr. It estimated that for 4 layers of 12 inch.times.12
inch (30 cm.times.30 cm) FLEX-PCB having eight 12 inches MSL and
eight 12 inches long strip lines. The approximated time to make the
trenches using the laser drilling is only 1.2 minutes. Aligning can
be done using infrared imaging analysis, which showed the metal
pattern (signal lines) in the opposite side of the dielectrics.
According to this invention, alternatively, trench can also be
opened using the wet (or dry)-etching after using standard
photolithography. For example, the polyimide such as Q-PILON,
marketed by Pi R&D Co, Ltd, Japan, can be used for High Speed
Flex-PCB. Standard IC fabrication technology can be used.
Alternatively, another method of polymer removal to open the
trenchm is using a milling machine. MITS Electronics, in Tokyo,
Japan markets the milling machine, which can make the drill in
dielectrics materials, manufactured for PCB. This machine has
control in the X, Y, and Z direction. The z direction accuracy of
this system is 0.1 mils. The instrument available in the market can
make the drilling automatically based on the trace designed. Using
the available drilling technology, the high speed FLEX-PCB can be
fabricated as noted in this invention
According to this invention, fabrication process for the microstrip
line type signal lines are described. The similar fabrication
process can be used for the high speed FLEX-PCB that has only
single layer of the signal lines, which are the microstrip type
configuration. Others layered can exist which may carry low speed
signal lines. In that case, other layers could be fabricated using
the uniform dielectrics as conventional FLEX-PCB fabricates. The
high speed FLEX-PCB could be hybridly stacked, in which single or
multiple layers could be dedicated using the non-uniform
dielectrics (dielectrics with air trenches).
(ii) Process for Stripline Type Signal Lines
FIGS. 7A, 7B, 7C, 7D, 7E, and 7F are process steps for building the
high speed FLEX-PCB with stripline type signal lines in accordance
to the present invention, wherein like parts are indicated by like
reference numerals, so that repeated explanation is omitted here.
Enlarged cross-sectional views of a portion of high speed FLEX-PCB
are only shown for explanation.
In the preferred embodiment, the process for the high-speed
FLEX-PCB having striplines type signal lines consists of signal
lines 16B formation in sheet material 34, opening the trenches 24B
under the signal lines 16B, formation of the trenches 24A (aligned
with signal lines 16B while stacked) in the sheet material 36
having uniform metal layer which acts as the ground 18, and
stacking of the sheet material 36 with trenches 24A and uniform
metal layer 18, sheet material 34 with trenches 24B and signal
lines 16A, and third sheet material 38 with uniform metal layer 18
and uniform core layer, with the help of the two adhesive layers
40, to form the multilayered high speed FLEX-PCB 42. The stacking
can be performed using the acrylic (called as the adhesive) 40 to
form the multilayered high speed FLEX-PCB 42 having stripline type
signal lines. The related process techniques for example the
patterning, trenches opening technique etc. are already explained
in FIG. 6, so that repeated explanation is omitted here.
According to this invention, fabrication process for the FLEX-PCB
with only stripline type signal lines are described. The similar
fabrication process can be used for the high speed FLEX-PCB that
has single or multiple layers of signal lines, which are the
stripline type configuration. Others layers may carry low speed
signal lines, which may consists of uniform dielectrics as
described in the Prior Art (FIG. 1 and FIG. 2). The high speed
FLEX-PCB could be hybridly stacked, in which single or multiple
layers could be dedicated using the non-uniform dielectrics
(dielectrics with air trenches).
(iii) Process for Multi-Layered High Speed FLEX-PCB
In the preferred embodiment as explained below, it is an object to
use the techniques as explained in FIGS. 6 to 7, in the off-chip
interconnects for multiple chip interconnection on the FLEX-PCB
(board). The board here considered is the board made from Polyimide
material or any other kind of dielectric material as mentioned
previously. Similar technique can be applicable for other
dielectric material board as explained earlier.
FIG. 8 shows the flow-chart of the high speed multilayered FLEX-PCB
fabrication process for the off-chip interconnects in accordance to
the invention, where in the like parts are indicated by the like
numerals, so that repeated explanation is omitted here. The
dielectric sheet (not shown) is made using the standard FLEX-PCB
technology for example using the slurry casting process. The slurry
is cast into about 200 .mu.m to 500 .mu.m thick ceramic sheets by
slip cast process. Each dielectrics sheet material 44 is the
conventional FLEX-PCB core layer 44. Metallization sheet 46 is made
using the conventional FLEX-PCB technology. After the
metallization, the trench or slot is opened in sheet 48 by using
the processes such as laser drilling, or dry-etching or wet-etching
(following patterning for etching) or mechanical drilling. Via
holes are formed through the dielectric sheet 44 by a punching
machine with punches and dies. A ceramic sheet 44 may have more
than 10,000 via holes in a 250 .mu.m square area. Low resistivity
conductor paste onto the punch sheet. In this process, via holes
are filled with the paste to form the contacts between the signal
lines. Low electrical resistivity material such as
silver-palladium, and gold instead of molybdenum or tungsten
refractory material can be used. The sheets are sintered at high
temperature, which makes lower electrical resistivity. The trenched
sheets 48 are precisely stacked in a pressing die in sequence by
the stacking machine. These sheets 50 are laminated together by hot
press. Density heterogeneities in the laminated samples influence
any shrinkage in the sintered substrate. Therefore, this lamination
process is homogenously carried out by means of the correct
dimensional die and punch with flat surfaces. Burn out and
sintering process for the multilayered FLEX-PCB board 52, may
necessary after lamination at the temperature suitable to ceramic
material used as the sheet. Additional via holes process (not
shown) are necessary to connect the signal lines located in
different layers.
(iv) Via or Micro-Via Structure in High Speed FLEX-PCB
In the preferred embodiment as explained below, it is an object to
provide the technique to design the via or micro-via in the high
speed FLEX-PCB, explained in FIGS. 6 to 8. This is one of the
techniques, can be used for the case of the high speed FLEX-PCB
with high-speed signal lines where opened trenches are used to
reduce the effective dielectric constant and also to reduce
effective tangent loss of the interconnects system. Any kinds of
the board materials such as Polyimide and other kind of the
dielectric material as mentioned previously can be used as the
FLEX-PCB material.
FIG. 9 shows the schematic showing the enlarged cross-sectional
view of a multilayered high speed FLEX-PCB with the high speed
signal lines and micro-via embedded into the FLEX-PCB in accordance
to the invention, where in the like parts are indicated by the like
numerals, so that repeated explanation is omitted here. According
to this invention, via or micro-via 54 can be formed without
damaging a board 56 and to have sidewalls to deposit copper. To
form the via or micro-via 54, the air-cavities (opened trenches) 24
A and 24B are needed to stopped at some reasonable distance of l
(shown in FIG. 9) which is dependent on the design rule tolerant.
The signal line connecting to the via or micro-via 54 consists of
two sections, (i) signal line 58 with opened trenches and (ii)
signal line 60 without opened trenches prior to the via or microvia
54. The impedance of the signal lines 58 and 60 are maintained at
the desired impedance by modifying the strip line width (not shown
in FIG. 9). In this case, as the signal line 60 having the distance
of l has the continuous dielectrics, for the fixed characteristic
impedance (for example 50 ohm) the metal (signal line) width is
adjusted to be narrower than the metal (signal line) width of
signal lines 58 with opened trenches. The signal lines 58 and 60
located on core layer 12D are connected to core layer 12B through
the ground layer 18C which is etched back before being stacked.
According to this invention, the via 54 is drilled or laser out or
etched after core 12A, core 12B, core 12C and core 12D are stacked.
After the via is cut, copper is deposited forming the connection of
two signal lines located in two core layers 12D and 1B. Then core
layer 12E is stacked with interconnect defined to overlap and
connect to the via.
FIGS. 10 and 10B show the schematic showing the enlarged top views
of the signal line layout which is connecting to the via or
microvia in the case of multilayered high speed FLEX-PCB with the
high speed signal lines and micro-via embedded into the FLEX-PCB,
in accordance to the invention, where in the like parts are
indicated by the like numerals, so that repeated explanation is
omitted here. According to this invention, signal lines consisting
of signal lines 58 and 60 (located on the surface 62) may have
transition. The transition of the signal lines 58 with underneath
air-cavities (opened trenches) 24 (24A and 24B) to the signal lines
60 without air-cavities (opened trenches) prior to the via 54. For
smooth transition without reflection of the signal, the transition
length l.sub.3 is used. The shape of the transition could be
trapezoidal or circular or ellipsoidal (not shown).
FIGS. 11A, 11B, 11C, and 10D show the schematic showing the shape
of the opening trenches into the core layers pf the high speed
FLEX-PCB in accordance to the invention, where in the like parts
are indicated by the like numerals, so that repeated explanation is
omitted here. According to preferred embodiment, the opening can be
square shape 66, rectangular shape 68, trapezoidal shape 70, or
circular shape 72 or ellipsoidal shape (not shown) where the top
openings 74 can be wider or similar to bottom opening 76. Noted
here that the top opening 74 closer to the metal line (signal line
or ground) than the bottom opening 76. The widths w.sub.1, w.sub.2,
and w.sub.3 of the top openings 74 can be same or wider than the
bottom openings 76. The widths w.sub.1, w.sub.2, and w.sub.3 as
shown in FIGS. 11A, 11B, and 11C could be smaller, same or larger
than the signal lines width (not shown). In the case of the
ellipsoidal or circular shaped openings 72, the width of the bottom
openings can be smaller, same or larger than the signal lines width
(not shown). The height (or deepness) of the openings can be
adjusted based on the bandwidth requirements of the
interconnects.
In the preferred embodiment as explained below, it is an object to
provide some calculated data for the high speed interconnects,
explained in FIGS. 12 to 14. These are the explanatory graphs
showing the advantages of the techniques. For each of the
calculation as a FLEX-PCB material, polyimide is used to show the
performance improvement. As mentioned earlier, this invention
covers also all kinds of flexible dielectrics materials having
dielectric properties and can be used as the board material.
FIGS. 12A, 12B, and 12C show the estimated results for variation of
the tangent loss and dielectric constant as the function of the
dielectric removal for the interconnects with opened trenches, in
accordance to the invention. Noted here that all the estimated
results are for the conventional FLEX-PCB materials for example
polyimide or the dielectrics having the dielectric constant and
tangent loss (dielectric loss) of 4.0 and 0.02, respectively. The
variation of the dielectric constant and tangent loss from 4.0 and
0.02, respectively are due to the dielectric removal. The estimated
dielectric constant and dielectric loss are closer to the effective
dielectric constant and dielectric loss for the interconnects with
dielectric removals to open the trench. The effective dielectric
constant and effective dielectric loss for all dielectric removal
(100%) are thought to be equivalent to 1.0 and 0.0, respectively.
In the estimation, if not mentioned, the dielectrics removal width
is considered as the same as that of the signal line (metal) width.
If the width of the trenches is made wider than the signal line
width, the less removal is necessary for the achieved effective
dielectric constant and effective tangent loss.
FIGS. 13A and 13B, and FIGS. 14A and 14B show the estimated results
for variation of the tangent loss and dielectric constant as the
function of the dielectric removal for the interconnects having
opened trenches in accordance to the invention. The results as
shown in FIGS. 13 and 14 are for the conventional FLEX-PCB material
for example, Polyimide and also for the assumption as already
explained in FIG. 12, so that repeated explanation is omitted here.
According to this invention, as the effective dielectric constant
is reduced, it is necessary to design/adjust the metal (signal
line) width to keep characteristics impedance fixed. The signal
line width is needed to keep wider than that of the signal line
with no dielectric removal. FIGS. 13A and 14A are the metal width
variation as the function of the dielectrics removal for the
microstrip type and stripline type signal lines, respectively. 0.0%
dielectric removal indicates the conventional type interconnects
without opened trenches. 100% dielectric removal indicates the
signal lines with out dielectrics and effective dielectric constant
and effective dielectric loss are 1.0 and 0.0, respectively. FIGS.
13B and 14B are estimated results showing the dielectric constant
versus signal line width with the dielectrics thickness are the
parameters for the microstrip type and stripline type signal lines,
respectively. All the results shown here are for the interconnects
with 50 ohm characteristics impedance. As depicted, to keep
characteristics impedance fixed for example 50 ohm, either signal
line width is needed to design wider or the thickness of
dielectrics is needed to be thinner than the interconnects without
opened trenches.
FIG. 15 compares the frequency responses of the preferred
embodiment in according to the invention. As explained in FIGS. 12
to 14, all estimated results are for the Polyimide materials, as
the conventional FLEX-PCB material. Similar approach covers also
other dielectric materials, which could be used as the FLEX-PCB
material. According to this invention, the interconnects can be
designed with controlled bandwidth by removing the appropriate
dielectrics from the interconnects. As depicted, based on the
percentage of the dielectric removal, the bandwidth can be
increased to 20 GHz and above.
FIG. 16A is the top view and FIGS. 16B and 16C are cross-sectional
views along AA' and BB' directions of FIG. 16A in accordance to the
present invention wherein the like parts are indicated by the like
numerals, so that similar explanations are omitted here. In the
preferred embodiment, two chips interconnection on the FLEX-PCB 77
are shown. As an example, processor 120 and memory 130
interconnection on FLEX-PCB 77 are shown as an example, and it
comprises with high-speed signal lines 78, core layers 80, adhesive
to stack the several core layers 82, and the ground (power line)
84. The core layers have the opened trenches 86, based on whether
they carry the high-speed signal lines. The high speed signal line
78 can be taken from the top of the FLEX-PCB layer and lower speed
signal line can be brought to the lower layer. This would reduce
the possibility any discontinuities, which may arise due to the
vias. Bandwidth of the interconnects using the technique as
mentioned previously, can be attained and thereby on-chip's signal
speed can be preserved. For simplicity in drawing, enlarge portion
of cross-sectional views for high speed (e.g. processor and memory)
chips portion interconnects are only shown. Complete FLEX-PCB
portion with considering lower speed chip interconnects are not
shown.
FIG. 17A is the top view and FIGS. 17B and 17C are enlarged
cross-sectional views along AA' and BB' directions of FIG. 17A in
accordance to the present invention wherein the like parts are
indicated by the like numerals as used in FIGS. 4 and 16, so that
similar explanations are omitted here. In the preferred embodiment,
two chips interconnection are shown. In the preferred embodiments,
alternatively, the high-speed chips interconnect in the separate
board 88, act as the for multi-chip-module. For example for
connecting the processor and memory, board with back-trench or slot
can be used and they can be fabricated using the process along with
the design as explained in FIGS. 6 to 10. Each board has the pins
90 coming out from the outside of the FLEX-PCB board 88 which can
be mountable on to the motherboard made from the conventional
FLEX-PCB materials for more integration and for ground/power and
low speed connections.
The dielectric materials include all kinds of ceramic materials
such as Duroid, PTFE, AlN, Al.sub.2O.sub.3, Mullite
(3Al.sub.2O.sub.3: 2SiO.sub.2), SiC, SiO.sub.2, Silcion nitride,
Silicon-Oxy-Nitride, BeO, Cordie-rite (magnesium alumina silicate),
BN, Glass (with different compositions), polyamide, epoxy glass,
CaO, MnO, ZrO2, PbO, alkali-halide (e.g. NaBr, NaCl) etc.) etc.,
and all kinds of the polyimides and benzocyclobutenes (BCBs) having
dielectric properties. Polymer dielectric material also includes,
but not limited to, Teflon, liquid crystal polymer, epoxy,
parylene, silicone-polyimide, silicone-gel, and fluorinated
ethylene propylene copolymer. It also includes materials of
elastomers (e.g. silicone elastomer), monomers, and gels. All
standard polymers can be available from the standard manufacturer
for example, Du-pont, Nelco, General Electric, Isola,
Hitachi-Chemical, Mitsui, and Mitsubishi-Chemical Industries.
Gore-Tex, Japan, markets liquid crystal polymer.
According to this invention, flow or no-flow type adhesive can be
used for stacking the multiple core layers with signal or ground
lines. It is highly desirable to use thinner adhesive in order to
get maximum performances advantages. For adhesive materials,
conventional available adhesives, marketed Dupont, etc. can be
used. The adhesives type could be flow or no-flow type based on the
pressure and temperature of the process during the stacking the
core layers. In order to avoid complete prevention of the adhesive
from flowing into the trenches, no flow type adhesive can be used.
By process optimization the trenches can be made to open as
designed and the designed response can be made to as close to the
experimental response.
In the preferred embodiments, details process condition has not
been described. However, it would need to optimize the process
condition to achieve the maximum performance. Absorption of the
water during the process may occur. High temperature annealing may
necessary before stacking to remove the water molecules as absorbed
during or after the process. The water resistant-coating can be
used on the trench surface after trench opened (and before
stacking) to prevent the water or gas absorption during the
process, which may reduce the reliability.
In the preferred embodiments as explained in FIGS. 12 to 15, only
the Polyimide based FLEX-PCB design parameters are shown as an
example. These results has been shown in an intention to show the
design ways for the interconnects according to this invention.
Optimized design parameters may needed based on the materials
parameters and interconnects structure and these can be achieved
using the three-dimensional (3-D) field solution. For other
dielectrics based FLEX-PCB (whether rigid or flex) similar design
ways can be used for achieving the maximum performance.
In the preferred embodiments as explained in FIGS. 4 to 17, each
core dielectric (sheet material) consisting of the dielectric,
adhesive and copper layer is considered for simplicity in
explanation and drawings. This invention also covers the FLEX-PCB
build-up made from the core consisting of the copper layer,
dielectric and adhesive. In this case, the process is the same as
explained earlier. Only difference is to open the back-trench which
passes from polyimide (all portion) and dielectrics (percentage as
necessary for bandwidth) (not shown here). For adhesive materials,
conventional available adhesive, marketed by Dupont. etc. can be
used. The adhesive type could be flow or no-flow type based on the
pressure and temperature of the process during the stacking the
core layers. In order to avoid complete prevention of the adhesive
from flowing into the trenches, no flow type adhesive can be used.
By process optimization the trenches can be made to open as
designed and the designed response can be made to as close to the
experimental response.
In the preferred embodiments as explained in FIGS. 4 to 17, only
strip line and microstrip line configurations are considered.
However, in accordance with the present invention, other signal
lines, not mentioned here, such as coplanar line configuration with
single or multiple signal lines (as single or differential) also
include. Dielectric coverage (not shown) using of the same or
different dielectric material can also be used.
In the preferred embodiments as explained in FIGS. 4 to 17, the
ground plan is located close proximity to the prepreg and the
opened trench (in the case of the strip-type and microstrip type
lines). This invention can also covered for the ground plan not
located under (and over) the trench openings. The ground plan can
be located both sides of the opened-trenches.
According to this present invention, alternatively. The ground plan
can be located both vertical sides of the opened-trenches to reduce
the interference.
The present invention has been described above by way of its
embodiments. However, those skilled in the art can reach various
changes and modifications within the scope of the idea of the
present invention. Therefore it is to be understood that those
changes and modifications also belong to the range of this
invention. For example, the present invention can be variously
changed without departing from the gist of the invention, as
indicated below.
According to the present invention, it is the object to provide the
high speed FLEX-PCB with interconnects having the opened trenches
for reducing the microwave loss for increasing the bandwidth of the
interconnects. It is also the object to use any dielectric material
(including conventional dielectric material and the manufacturing
technology) in the technique and could increase the bandwidth
tremendously. In simplicity of drawing, preferred embodiments are
described mostly considering the microstrip line and strip line
configurations. However, all line configurations such as coplanar
line with single or multiple signal line (including differential
line) also cover this invention.
According to the present invention, high speed FLEX-PCB with
interconnect system uses inhomogeneous dielectric system consisting
of the dielectrics and the portion of air layer to reduce the
effective dielectric loss and dielectric constant, wherein the
inhomogeneous dielectric system has two or more dielectrics, and
one of them dielectrics has lower dielectric loss and dielectric
constant. In the preferred embodiment, opened trench with air is
used in the high speed FLEX-PCB. Alternatively the low dielectric
loss (and/or dielectric constant) material or the liquid crystal
polymer fills up the trench.
According to this present invention, the dielectric and tangent
loss variation are estimated based on the assumption that the field
is accumulated under the signal lines, to show the advantages of
the preferred embodiments and to make it easy in estimation. In
fact, the electrical field is spread outside the signal line. More
dielectric constant and dielectric loss variation are possible if
the trench width is wider than the signal line-width, and they can
be extended in both sides of the trench.
The present invention is described here, considering only onto the
high-speed electrical signal. However, the present invention can be
also used in the interconnects system where both electrical and
optical signal can be transmitted using the same signal line. For
example, the trench portion is used to reduce the effective
dielectric loss and effective dielectric constant. By using the
opened backside slot or opened trench the signal is mostly flowing
through the trench filled up with air or lower dielectric loss
material. In the interconnects where both high speed electrical
signal and high speed optical signal are considered, the trench or
backside slot used can be used for transmitting the optical and
electrical signal together, and significant bandwidth of the
interconnects system with high integration capability can be
realized.
Several preferred embodiments for high-speed on-chip and off-chips
interconnects are described considering the microstrip line
configuration and also the dielectric system with back-trench or
slot. All signal line configurations as mentioned earlier covers
under this invention. The shape of the trench could be any type
such as square, rectangular, circular, trapezoidal or any
polynomial shape, or any shape convenient for manufacturing. These
can be filled up by dielectric material having the lower dielectric
constant than the dielectric substrate.
Although the invention has been described with respect to specific
embodiment for complete and clear disclosure, the appended claims
are not to be thus limited but are to be construed as embodying all
modification and alternative constructions that may be occurred to
one skilled in the art which fairly fall within the basic teaching
here is set forth.
The present invention is expected to be found practically use in
the high-speed on-chip, off-chip interconnects, where the signal
speed 5 Gb/s and beyond are necessary using of the conventional
material, and the bandwidth of the interconnects can be made
ideally to speed of the light for no-loss transmission line. The
present invention can also be implemented in the high-speed single
or multiple signal connectors, and high-speed cables (not shown).
The applications include on-chip interconnects where high-speed
electronics chips or electronics chips with optical chips are need
to be connected. As ideally the bandwidth of the interconnect
system can be made to close to fiber, future monolithic (and also
hybrid near future) integration of electronics and optical chips
can also interconnected without (much or none at all) sacrificing
the chips speed. The application also includes the high speed
multichip module interconnection, 3-D chip or memory
interconnection, high speed parallel system for computer animation
and graphics for high speed 2-D or 3-D video transmission, and high
bandwidth image display, high speed router where high speed
electronics switches (or IC) are needed to be interconnected. The
application also include the high speed (5 Gb/s and beyond)
connectors and cables for high speed board-to-board, rack-to-rack
interconnection, and also single or multiple high-density signal
connections and carrying from one side to other in longer path.
* * * * *